1. Field of the Invention
The present invention relates generally to detection apparatuses and, more particularly, to an apparatus for detecting neutrons. More specifically, the invention relates to a high-efficiency threshold detector of fast neutrons.
2. Prior Art
Neutron counters using materials sensitive to neutron-induced fission are known in the art. As a simple example, a counter sensitive to thermal neutrons could be constructed by depositing a thin coating of .sup.235 U metal on the interior wall of a gas-filled proportional counter. The .sup.235 U deposit should be thin enough that the primary fission fragments can escape the metal and be detected in the proportional counter.
A neutron counter that is sensitive to fast neutrons can be made by using a material with a higher threshold for neutron-induced fission, like .sup.238 U. The cross section for neutron-induced of .sup.238 U rises rapidly from about 2.5.times.10.sup.-4 b (where 1 b or barn=10.sup.-24 cm.sup.2) at 0.5 MeV (megaelectron volt) to over 1000 times greater at energies above 1.5 MeV. Therefore, a counter using .sup.238 U as the sensitive material would only be sensitive to neutrons with incident energies above this cross section threshold, i.e., a "threshold detector".
A typical high-efficiency fast neutron detector first thermalizes the neutron by multiple scattering in a hydrogenous material, then detects the thermal neutrons in a proportional counter. All spectroscopic information is lost in the thermalization process, as is all information about the direction from which the neutron was incident upon the detector. These detectors typically have efficiencies of at least a few % for neutrons with energies in the hundreds of keV (kiloelectron volt) to the MeV range, or "fast" neutrons. Typical fast neutron spectrometry techniques, such as neutron time of flight, proton recoil telescopes, etc., suffer from efficiencies that are many orders of magnitude below this. Spectrometers using .sup.10 B doped scintillators have been developed that achieve efficiencies in the few % range. However, the energy resolution is rather poor, being 55% to 33% full width at half maximum (fwhm) in the range from 0.5 to 5.7 MeV.
The fast neutron detection approach of the present invention avoids the multiple collision process of thermalization, and absorbs all of the neutron energy in one step via the fission reaction. There are a number of heavy elements that have relatively high, fast-neutron-induced fission cross sections. For example, .sup.236 U, .sup.234 U, .sup.238 U, .sup.241 Am have fission cross sections for fast neutrons in the range of one barn. However, this is not an exhaustive list. Materials that meet the criterion of having a significant cross section for neutron-induced fission only for neutrons with energies above about 100 keV will be referred to hereafter as fast-fissionable material (FFM).
A problem is immediately apparent when the nature of the fission reaction is considered. When a fast neutron (a few Mev) induces a fission of a FFM, several hundred MeV of energy is released. Most of this energy is divided between two fission fragments; the rest is given to an average of three or more neutrons and released as gamma-rays. This neutron multiplicity more or less eliminates this system's usefulness as a neutron spectrometer.
It does not, however, eliminate the possibility of using it as a threshold detector for a yes/no discrimination between fast and slow neutrons. As an example, the reaction threshold for neutron-induced fission of .sup.238 U is about 1.5 MeV, at which point the fission cross section rises sharply by a factor of 1000. This barrier will effectively discriminate between fast and slow neutrons. This discrimination would be further enhanced if a detector exploiting this reaction is also shielded by a thin layer of plastic heavily doped with .sup.10 B or some other thermal neutron shield. Then, any thermal neutrons present would be greatly attenuated without seriously affecting the fast neutron flux being measured.
Passive variations of this idea have been done in the past by backing a fissionable foil with a film to record the tracks left by the fission products, and then counting the tracks after etching the film. Also, active fission fragments detectors have been used. These detectors use only very thin deposits of fissionable material so that the short range, on the order of microns, primary fission fragments will be able to escape from the fissionable material and be detected. This greatly limits the efficiencies for fast neutron detection that these detectors can achieve. Some examples, of the latter type of detectors can be found in U.S. Pat. Nos. 3,140,398, to Reinhardt et al.; 3,878,108, to Burgkhardt et al.; and 4,804,514 and 4,857,259 both to Bartko et al.
The neutron dosimeter of Reinhardt et al., for example, uses a composite foil of a nickel disk electroplated with a mixture of .sup.235 U, .sup.237 Np and .sup.238 U in specific proportions, to detect the neutron flux of a typical fission spectrum. The plated surface of the foil is mounted on a solid state surface barrier detector.
Burgkhardt et al. discloses a dosimeter to be worn on a finger which includes a fissionable foil of thorium or neptunium covered on one or both sides by one or more flexible detector foils made of a polycarbonate resin.
The two Bartko et al. patents describe neutron dosimeters which use different techniques to detect the ions released from a thin layer of fission material struck by neutrons. In the U.S. Pat. No. 4,804,514, the ions induce light pulses in a scintillator material disposed adjacent to the fission layer, and a photomultiplier converts the light pulses to electrical pulses. In the U.S. Pat. No. 4,857,259, a layer of fission material is disposed adjacent to a layer of material which changes its electrical conductivity in accordance with the density of implant ions from the fission material. A measurement of the conductivity provides information for determining neutron dose.